In such drying systems, including spray drying systems, there are means for transporting powdery material from powder recovery units such as cyclones and bag filters downstream of the drying chamber to upstream units such as to relevant positions of the drying chamber itself or to a finishing treatment unit such as a vibrating fluidizer or integrated/static fluid bed for handling the material discharged from the drying chamber. Typically, such conveying lines are configured to transport powdery material in the form of fines, that is, powdery material of satisfying composition but of a size and/or configuration that needs further processing in order to fulfil quality requirements, i.e. a so-called fines return system, during normal operation, and the same conveying lines are often used for transporting powdery material to the drying chamber and for transporting powdery material to a finishing treatment unit by having one or more multiple diverter valves in line with the conveying lines. Such diverter valves will direct the flow from the powder recovery units to one of a number of destinations. The conveying lines leading from the diverter valve to non-active destinations are thus empty. These non-active conveying lines are known as idling lines. Due to the high temperatures, which are commonly present in spray drying systems, the idling lines have a lower temperature than the active lines, and as such any moisture in the air, will condensate in the idling lines. Thus, when switching the diverter valve to activate a formerly idling line, the powdery material will enter a moist section, which is to be avoided for the hygroscopic and sticky characteristic reasons mentioned above as well as hygienic and bacterial reasons. A common solution to this problem is to add a purge gas valve after each exit of the diverter valve, as close to the valve as possible, in order to run purge gas through the idling lines in order to remove the condensation. This solution, while alleviating the problem somewhat has some limitations as the purge gas valve can never be placed close enough to the diverter valve, in order to remove all moisture. As such, some moisture will be present.
Cleaning requirements for the drying and powder handling equipment, i.e. the so-called dry areas, can be met by either dry or wet methods, or a combination of both. Dry methods involve manual sweeping of surfaces in contact with the product, or air sweeping by allowing a high velocity air stream to pass over the surfaces in question. Wet methods involve either manual hosing of surfaces or more effectively by use of automated cleaning-in-place systems (CIP systems).
The very nature of e.g. dairy and food products means that the possibility of product deposits formation on the surface of the process equipment is always real. The products' fat, sugar and protein content, gives hygroscopic and sticky characteristics at the temperature and humidity conditions present during processing. For drying plants in general, it is often relevant and profitable to equip the plants with automatic CIP in order to reduce the downtime of the plant. The risk of deposit formation increases with the running time of the plant. Industrial CIP systems can clean individual items or complete process plants, involving integrating cleaning nozzles into the plant components together with their associated pipe work, and instrumentation.
The complex nature of the cleaning regimen is particularly pronounced in spray drying systems comprising a fines return system. In such systems, it is of course vital that the conveying line for the stream of powder is thoroughly dried by air or other gas after wet CIP to ensure that the stream of powder in the conveying line is not compromised by any remains of moisture.
Although such CIP systems provide a thorough cleaning, it is also a challenge that the CIP liquid needs to be drained and the parts involved need to be dried very thoroughly in order to avoid the risk of contamination and subsequent sticking of product. It is not uncommon that cleaning regimens do not involve all parts of such a spray drying system to be cleaned by CIP. This applies in particular to the dry sections of the system, including typically the fines return system.